A new City Council investigative unit has opened a wide-ranging inquiry into the city agency that regulates the private carting industry — the second instance in four months where the Council’s investigators have challenged a mayoral agency under Bill de Blasio.

The Council was concerned about the way the agency, the Business Integrity Commission, carried out its mandate because of irregularities in the waste hauling industry, including the deaths of two people in recent months who investigators determined had been struck by trucks operated by Sanitation Salvage, a large commercial trash hauler.

On Friday, Councilman Ritchie Torres, the chairman of the Council’s committee on oversight and investigations, sent a letter to Daniel D. Brownell, the Business Integrity Commission’s chairman, asking for a list of documents and other materials relating to the commission’s activities.

The Council’s new tone reflects the desire of the Council speaker, Corey Johnson, to demonstrate the legislative body’s independence and his intention, along with Mr. Torres, to expose poor management and wrongdoing, and make the workings of government less opaque.

Now the Council is starting to flex its muscle.

In April, Mr. Johnson hired Steve Pilnyak, a financial crimes prosecutor from the Manhattan district attorney’s office, to lead the unit. Mr. Pilnyak has been quietly building up his staff, which now includes several lawyers and accountants, a former F.B.I. investigator and even an astronomer with an expertise in analyzing complex sets of data.

“Never before has the Council sought as much information and documentation from an agency as we are seeking from B.I.C.,” Mr. Torres said, referring to the Business Integrity Commission. “It’s a top-to-bottom review of all the agency’s budget and structural practices and personnel.”

The mayor’s office announced on Friday that the commission had suspended the license of Sanitation Salvage over safety concerns.

Natalie Grybauskas, a spokeswoman for the mayor, said that the timing of the commission’s action was not related to the Council’s request for documents.

Mr. de Blasio said recently that he wanted to work with the Council to give the commission “stronger powers of enforcement,” adding, “I think B.I.C. does a good job but also needs better tools.”

The Council’s new investigative unit has also butted heads with the New York City Housing Authority, and Mr. Torres raised the possibility of issuing a subpoena to force it to hand over the documents, something that he said the Council has not done since 2005.

In May, Mr. Torres and another Council member, Alicka Ampry-Samuel, who leads the committee on public housing, sent a nine-page letter to the housing authority, also known as Nycha, demanding an extensive series of documents relating to heating breakdowns last winter and to its now-notorious failure, over several years, to conduct lead paint inspections.

The materials requested included documents relating to Nycha’s false certifications that it was in compliance with federal lead paint inspection rules and others relating to how members of Mayor Bill de Blasio’s administration had learned of the lapses.

“Among the information we are seeking is communications between Nycha and City Hall,” Mr. Torres said. “The essential question left unanswered is: Who in City Hall knew what, when?”

Mr. Torres said that Nycha initially told the Council it had lost the letter, which was re-sent in June. He said that Nycha then agreed to deliver most of the documents requested by Aug. 20. But so far, he said, Nycha has not provided the documents. On Aug. 20, instead of handing over the material, Nycha asked for more time.

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Senator Kirsten Gillibrand, left, with Councilwoman Alicka Ampry-Samuel, who leads the City Council’s housing committee and has requested documents from Nycha.CreditHolly Pickett for The New York Times

“There’s a point at which these extensions become a delaying tactic,” Mr. Torres said, “and even though the Council prefers conciliation as a general rule, we refuse to be strung along indefinitely. To be blunt, my committee will issue a subpoena as a final recourse.”

He added: “A willful lack of cooperation from an agency is an assault on the legitimacy of the City Council as an oversight institution and that’s something we cannot and will not abide.”

Robin Levine, a Nycha spokeswoman, said that the Council’s letter was not lost, but that officials had asked the Council to provide an emailed copy in addition to the original paper copy.

She added that Nycha had been in weekly contact with Council investigators and that Stanley Brezenoff, the acting chairman of the authority, “is in the process of reviewing the large volume of documents requested by the Council.”

“We have new leadership at Nycha that is committed to creating a culture of compliance and transparency, and who take our obligation to provide accurate information to any oversight entities seriously,” Ms. Levine said.

While Mr. Johnson and Mr. Torres can be brash, the man that Mr. Johnson chose to run the council’s investigations unit, Mr. Pilnyak, is soft-spoken and self-effacing.

“He is impressively levelheaded and even tempered,” Mr. Torres said. “I speak loudly and carry a big stick. He speaks softly and carries a big stick.”

Mr. Pilnyak, 40, was born in Ukraine when it was part of the Soviet Union. He emigrated to the United States with his family when he was 3.

He grew up speaking Russian at home and hearing stories from his parents and grandparents about the country he left behind — where there was no rule of law and citizens had few rights. He said those stories instilled in him a desire for justice.

Out of law school he got a job at Kaye Scholer, a corporate law firm. But he wanted to be a prosecutor, he said, and after a few years he took a position in the Manhattan district attorney’s office. He did not tell his parents or his grandparents until he had already given notice at his old firm.

“When I did, they cried,” he said. As immigrants who had known hardship, they could not believe he would trade his high salary for the lower pay of a public servant, he said.

Mr. Pilnyak spent 10 years at the district attorney’s office, working in the trial bureau and then in the major economic crimes bureau, where he became a supervisor of the financial intelligence unit. He played a lead role in prosecuting executives at the law firm Dewey & LeBoeuf for accounting fraud. He said the work satisfied his sense of justice. “The client is whatever the truth is,” Mr. Pilnyak said.

Mr. Pilnyak, baby faced with a shock of black hair, said that his role at the Council was to look at “systemic issues of waste and efficiency,” adding: “We’re there to ensure that city agencies and the mayor are living up to the work they’re supposed to.”

Christopher Conroy, his boss at the district attorney’s office, said that Mr. Pilnyak was well suited to his new job.

“He is very careful in terms of how he approaches an issue or a set of facts. He has very good instincts about where to look or who to talk to when trying to dig out information,” Mr. Conroy said. “He really had a strong sense of holding people accountable, whoever they were and wherever they sat, for the wrongdoing they engaged in.”

A version of this article appears in print on , on Page A16 of the New York edition with the headline: City Council Has a New Investigative Muscle. And It’s Starting to Flex It.. Order Reprints | Today’s Paper | Subscribe